Composite laminar structures are strong and light-weight. Their use is well known and they are frequently used in automotive, aerospace, sporting goods and marine applications.
Typically composite materials are manufactured by stacking layers of a fibrous reinforcement material which is preimpregnated with a curable resin material (prepreg). The resin material is then cured by heating the stack whilst it is being compressed. This causes the resin to flow to consolidate the fibrous stack, and then to subsequently cure. This results in an integral laminar composite structure.
Composite materials can also be formed by arranging layers of dry fibrous material into a mould and then infusing with a curable resin. The resin wets out the fibres of the dry material before being cured. This process is known as resin transfer moulding (RTM).
Both methods result in a composite material with a laminar structure having a series of layers of impregnated fibrous reinforcement.
Both methods rely on the use of curable resin materials, i.e. polymeric materials that can be cured to form an interpolymer network by the addition of energy, particularly by heating. The cure cycles employed for curing components comprising curable resins are a balance of temperature and time, taking into account the reactivity of the resin. From an economic point of view it is desirable that the cycle time is as short as possible. In many cases it is also preferable for curing to take place at relatively low temperatures, such as 80° C., or even lower. As well as requiring heat to initiate curing of the resin the curing reaction itself can be highly exothermic and this needs to be taken into account in the time/temperature curing cycle. It is important to balance the needs for short moulding cycles employing reactive resins and controlling the exotherm to avoid damage to the resin and/or the mould. Curing agents (sometimes referred to as “curatives”) are therefore often used in order to activate and control the curing of resins to provide the required cure cycle, the exotherm of the cure and the properties of the final cured resin, and/or to reduce the temperature at which curing can be carried out. A wide range of curing agents for resins have been proposed and are widely used.
The outlife of a resin is the duration for which the resin can be stored without it cross linking to a point where it is no longer useable. The desire for a short cure cycle and/or relatively low temperature curing must also be balanced with the outlife of the resin. The cure cycle and/or effective curing temperature can be reduced by adding more curing agents and/or curing accelerators; however this compromises the outlife of the resin. This problem is sometimes addressed by storing and transporting the curative and the resin separately, and mixing them only shortly before curing, but this introduces additional processing steps, and may also lead to poor curing due to inefficient mixing of the curative in the resin.
Various attempts have therefore been made to incorporate curatives in rein compositions, but to restrict the interaction of the curative and the resin under normal storage conditions. One such approach is to store resins containing curatives, or products comprising such resins, at reduced temperature, however this leads to difficulties in storing, transporting and handling the resins and resin infused products. Alternative approaches include modifying the curative so that it is not capable of interacting with the resin under normal storage conditions, or so that the rate of curing is reduced under normal storage conditions, for example by encapsulating or coating the curative to reduce interaction with the resin. However, the production of fully coated or encapsulated curatives may be difficult, and if the coating or encapsulation is not complete, or if it becomes damaged after formation, the curative may be rapidly released, leading to unpredictable curing of the resin. The materials used to form the coating or encapsulation may also adversely affect the properties of the cured resin.
EP0672707 is directed to microencapsulated curing agents for thermosetting resin compositions in which a curing agent for a thermosetting resin is microencapsulated in a thermoplastic resin dissolvable in the thermosetting resin. However, the process by which the curing agent is microencapsulated is complex, and the curing agent may be released unpredictably if the microencapsulation is not complete and/or if the microencapsulation coating becomes damaged after formation. Additionally, the thermoplastic resin and/or other components required to form the microencapsulation coating may adversely affect the properties of the thermosetting resin during or after curing.